The "vent only" designation is because of the sharp bend. A drain elbow has a wider radius and is better because debris can (does) jam up in a sharp corner. It will work is the strick sense of the word, but I would avoid it. You want to have as straight a shot from the fixture to the drain as possible, and "soften" the bends that are necessary. I think you'll have problems with the vent-only elbow, especially for a toilet.